The Enforcement Acts, among other powers, allow the president to call up military forces when state authorities are either unable or unwilling to suppress violence that is in opposition to the constitutional rights of the people.
https://en.wikipedia.org/wiki/Posse_Comitatus_Act
The President may employ the armed forces … to … restore public order and enforce the laws of the United States when, as a result of a natural disaster, epidemic, or other serious public health emergency, terrorist attack or incident, or other condition … the President determines that … domestic violence has occurred to such an extent that the constituted authorities of the State or possession are incapable of maintaining public order … or [to] suppress, in a State, any insurrection, domestic violence, unlawful combination, or conspiracy if such … a condition … so hinders the execution of the laws … that any part or class of its people is deprived of a right, privilege, immunity, or protection named in the Constitution and secured by law … or opposes or obstructs the execution of the laws of the United States or impedes the course of justice under those laws.[8]
https://en.wikipedia.org/wiki/Posse_Comitatus_Act